search(postgres): _lastUpdated returns 500 on every prefix and precision

[angela-helios]

Found in the #448 Postgres leg. Every _lastUpdated search fails with an opaque 500 on the Postgres backend — all precisions (2026-09-01, 2026-09-01T00:00:00Z) and all prefixes (eq/gt/ge/le):

GET /Patient?_lastUpdated=2026-09-02          → 500
GET /Patient?_lastUpdated=gt2026-09-01T00:00:00Z → 500

Server log:

ERROR helios_rest::error: internal error while processing request
  error.detail=internal error in postgres: Failed to execute search: error serializing parameter 2

Cause

build_last_updated_condition (crates/persistence/src/backends/postgres/search/query_builder.rs:645) binds the raw query string as a text parameter directly against the last_updated column:

conditions.push(SqlFragment::with_params(
    format!("last_updated {} ${}", op, param_num),
    vec![SqlParam::text(&value.value)],
));

last_updated is TIMESTAMPTZ, and tokio-postgres refuses to serialize a text param into it — hence "error serializing parameter 2" on every query.

Two layers to fix:

1. Bind a real timestamp (or cast) so the query executes at all.
2. Apply the same precision-range semantics the date-parameter path already has (the range helper at the top of the same file): eq at day precision must mean >= day AND < day+1, not a text =. The fix(search): normalize sqlite date comparisons across precisions #463 date fix explicitly covered _lastUpdated's separate code path on SQLite; Postgres needs the equivalent.

SQLite answers the same queries correctly. The #448 battery runs 22 checks on this backend; these are the only 2 failures.